Eric Larour

Rheology of the Ronne Ice Shelf, Antarctica, Inferred from Satellite Radar Interferometry Data using an Inverse Control Method

[1] The Antarctic Ice Sheet is surrounded by large floating ice shelves that spread under their own weight into the ocean. Ice shelf rigidity depends on ice temperature and fabrics, and is influenced by ice flow and the delicate balance between bottom and surface accumulation. Here, we use an inverse control method to infer the rigidity of the Ronne Ice Shelf that best matches observations of ice velocity from satellite radar interferometry. Ice rigidity, or flow law parameter B, is shown to vary between 300 and 900 kPa a 1/3 . Ice is softer along the side margins due to frictional heating, and harder along the outflow of large glaciers, which advect cold continental ice. Melting at the bottom surface of the ice shelf increases its rigidity, while freezing decreases it. Accurate numerical modelling of ice shelf flow must account for this spatial variability in mechanical characteristics. Citation: Larour, E., E. Rignot, I. Joughin, and D. Aubry (2005), Rheology of the Ronne Ice Shelf, Antarctica, inferred from satellite radar interferometry data using an inverse control method, Geophys. Res. Lett., 32, L05503, doi:10.1029/2004GL021693.

Larsen B Ice Shelf rheology preceding its disintegration inferred by a control method

A new, complete velocity field from satellite remote sensing is combined with numerical modeling to infer the rheology of the Larsen B Ice Shelf before its disintegration. The resulting spatial distribution of the flow parameter exhibits large variability, which reflects very well observed ice shelf features. This variability is explained by factors including advection of colder ice from tributary glaciers, bottom melting, and the presence of zones of strong shear and fracture. The inferred distribution is applied to simulate numerically the flow regime ofthe ice shelf and to examine its modification by the presence of open rifts and by the retreat of ice shelf front between 1996 and 2000. Results demonstrate that variable rheology is essential to understanding ice shelf evolution, especially the close relationship among frontal retreat, fracture, ice flow acceleration, and the destabilization of ice shelves. Copyright 2007 by the American. Geophysical Union.

Modelling of rift propagation on Ronne Ice Shelf, Antarctica, and sensitivity to climate change

[1]xa0The calving of icebergs from large Antarctic ice shelves is controlled mainly by the formation and propagation of rifts originating from the side margins of the ice shelf and local areas of grounding. Using InSAR, we observe the evolution of rifts along Hemmen Ice Rise, on Ronne Ice Shelf, Antarctica prior to the large calving event of October 1998. We couple these observations with a computer model combining the viscous flow of an ice shelf with a linear elastic fracture mechanics description of the propagation of rifts. The model reveals that the ice melange trapped in between the rifts exerts a major control on the propagation of rifts, and in turn on ice shelf stability. Melting of the ice melange from oceanic or atmospheric warming would significantly increase the propagation rate of rifts and threaten the ice-shelf stability.

Processes involved in the propagation of rifts near Hemmen Ice Rise, Ronne Ice Shelf, Antarctica

Interferometric radar images collected by ERS-1, ERS-2 and RADARSAT-1 are used to observe the rupture tip of rifts that propagate along Hemmen Ice Rise on the Ronne Ice Shelf, Antarctica. Interferograms generated in 1992 and 1997 allow for the observation of ice deformation accumulated over 9 and 24 days respectively. These interferograms are combined, in order to separate the continuous process of creep deformation from the more cyclic motion caused by variations in ocean tide. An examination of local gradients in creep deformation reveals the pattern of ice deformation around and near the rupture tips and rifts with great precision (up to 10 cm a-1). We compare the observations with a deformation model for ice and obtain the following results: (1) The tidal oscillation of the Ronne Ice Shelf only yields small deformations along the rifts and near the rupture tips. (2) Along the ice front, the rifts and at the rupture tips, vertical bending is observed which is well explained by a model of viscous deformation of ice. Furthermore, the model indicates that the deformation pattern observed at the rupture tips is a sensitive indicator of the propagation state of the rifts (i.e. active vs inactive). (3) The viscous adjustment of ice is the dominant mode of deformation, masking the deformation pattern predicted by linear elastic fracture mechanics (LEFM). (4) Yet, at a spatial scale equivalent to the length of a rift, the propagation rate is well predicted by LEFM.

A thermo/opto/mechanical testbed validation using Cielo

Due to their scale, operating environment, and required levels of operating precision, the design of the next generation of space-based observatories will necessarily place an ever-greater reliance on numerical simulation. Since it will be impossible to fully ground-test such systems prior to flight, system-level confidence must come, in large part, from correlated subsystem tests, system-level simulation, and an overall design understanding based on quantification of margins of uncertainty, sensitivity analyses, parameter variation studies, and design optimization. Further challenges will necessarily arise due to the actively-controlled nature of such systems, requiring fundamentally-integrated thermal, structural, optical, and controls models. In this paper we will discuss Cielo, JPLs multidisciplinary, high-capability compute platform for systems analysis, and describe some of the challenges in demonstrating these capabilities for the first time on a complex model, the Space Interferometry Missions Thermal-Structural-Optical (SIM-TOM3) testbed. The successes and lessons learned from these activities have the potential to greatly influence subsequent test programs, leading to greater design understanding, improved mission confidence, and significant cost and schedule reductions.